Quantizer

Modular synths tend to follow a scheme were a specific change in voltage – such as 1.00 volts – results in a precisely one octave change in pitch. In a 1 volt per octave scheme, 1/12 of a volt change results in a semitone change in pitch.

It can be difficult to precisely set a knob (say, on a sequencer) to hit the exact voltage required to get the desired pitch. A quantizer auto-corrects the input voltage to the nearest desired target, such as the voltage that corresponds to a semitone or other note in a scale. These are occasionally built into modules like sequencers or oscillators, but quite often they are standalone modules.

You don’t have to use a quantizer just for pitch voltages; it can also be interesting to use one to convert a smoothly varying input voltage into a “stepped” voltage on output to create machine-like modulations and the such.